The present two-stroke gasoline engine is of the scavenging type wherein through scavenging ports, fresh pre-mixed fuel air mixture flows into the cylinder to scavenge the residual gas out of an exhaust port; two shortcomings exist in such scavenging process as follows:
First, during scavenging process, part of the fresh fuel-air mixtures serving as scavenging gas flow often directly escapes out of cylinder through the exhaust port, causing the so-called short-circuitting loss of unburned mixture. When the engine operates under high loading condition, such loss appears more serious.
Second, during scavenging process, the burned gas is not totally scavenged and mixes with the fresh fuel air mixture. After the scavenging process ends, part of the burned gas remains in the cylinder, and when the engine operates under low loading condition, because the scavenging gas flow is weaker, the scavenging efficiency is correspondingly lower, and consequently, the ratio of burned residual gas to fresh fuel air mixture is too high. Such too high ratio is extremely apt to cause misfire of the mixture and lower the combustion efficiency.
The above two shortcomings are the major reasons why great amount of hydrocarbon is contained in the exhaust of a two-stroke gasoline engine.